What Compression Reading Should I Get?

Discussion in 'Heads and Cylinders' started by exokinetic, Apr 24, 2016.

  1. exokinetic

    exokinetic New Member

    Mar 18, 2016
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    What reading on a compression gauge is correct for our 66/80cc China Girl Engine?

    Long Answer?

    It depends on a few factors.

    1. The Combustion Chamber Volume. Usually measured in CC's (I. E. Fred's 5.0, 5.5, 6.0, 7.0 CC Heads). The Stock China Girls heads, as far as I have seen, come in two flavors. An angle fire spark plug, with low (relatively) combustion chamber volume (I haven't measured it). A straight fire spark plug, with high (relatively) combustion chamber volume.

    If everything else was kept the same, the reduced combustion chamber volume of the angle fire head would result in a higher compression ratio, and thus, a higher reading on the compression gauge.

    Conversely, on the same engine, the increased combustion chamber volume of the straight fire head would result in a lower compression ratio, and thus, a lower reading on the compression gauge.

    2. The Piston Dome Volume. This is difficult to measure, and is usually provided by the piston manufacturer (balls!). If the piston had a flat top this measurement would be 0. A dome on the top of the piston (like our lovable China Girl) results in a positive measurement, in CC's, equal to the volume of the Dome Area above the piston crown. Due to loose manufacturing tolerances there are minor variations in this volume, witch would result in minor variations in the reading of a compression gauge if all that was changed was a piston of the same type.

    3. The Head Gasket Thickness. The head gasket lifts the head higher relative to the piston crown at top dead center. This means a thicker gasket will result in increased combustion chamber volume, reduced compression ratio, and thus, a lower reading on the compression gauge.

    3a. Special Tip: Different thickness head gaskets are often used to adjust compression ratio. This has the unfortunate effect of changing the squish gap, google "squish band" if you want to learn more.

    4. The Deck Height. This, in the most basic sense, is a measurement (mm or in) of the distance between the piston crown (the topmost edge of the piston) at top dead center, and the cylinder deck. If the piston crown is below the cylinder deck at top dead center, the measurement is positive and represents "positive deck" (because the deck of the cylinder is above the the piston crown. If the piston crown is above the cylinder deck at top dead center, the measurement is negative and represents "negative deck".

    The greater the Deck Height (or the higher the cylinder deck is above the piston crown at top dead center), the more area is left between the dome of the piston at top dead center and the head. This results in higher (relatively) combustion chamber volume, lower compression ratio, and thus, a lower reading on the compression gauge.

    Factors that effect Deck Height:

    A. Crank Stroke: If using the same piston and cylinder without modification, reducing crank stroke from say 40mm (long stroke) to 38mm (short stroke) would increase deck height by 1mm. 40mm - 30mm = 2mm. 1mm reduced piston travel at the top and bottom of its stroke. This would increase combustion chamber volume, reduce compression ratio, and thus, result in a lower (relatively) reading on the compression gauge.

    B. Connecting Rod Length: Even if the actual swept stroke measurement of the piston remains the same (same crank) if a 1mm shorter connecting rod is used, the Deck Height is now increased by 1mm. This results again, in increased combustion chamber volume, reduced compression ratio, and a lower (relatively) reading on the gauge.

    B1. A little secret. No two connecting rods (even if marked the same) are EXACTLY the same length. Even these seemingly tiny variances in length will have an effect on the reading on the compression gauge. If there is a tolerance variance large enough to change the position of the piston by 0.010" up or down, it will result in about a 10psi change in the reading on the compression gauge (for example, changing out a head gasket for one that is 0.010" thicker or thinner)

    C. Cylinder Height. The height of the cylinder, measured from the bottom face that seals to the crankcase, to the top face that seals to the head. A 1mm taller cylinder than your current one (I have seen MANY different heights in the wild) will result in a "taller" deck height. This will again result in increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

    D. Cylinder Base Gasket. The cylinder base gasket (obviously) lifts the cylinder by an amount equal to the thickness of the gasket. A thicker cylinder base gasket will lift the cylinder, resulting in a taller deck height, increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

    E. Piston Wrist-pin Location.The distance between the piston wrist-pin hole and the crown of the piston. There are obviously two standard piston wrist-pin locations we have in our China Girl engine, the type A and the type B. If everything else stays the same, and you switch to a piston with a higher wrist-pin location (relative to the piston crown) the piston will be lower in the cylinder at top dead center resulting in taller deck height, increased combustion chamber volume, reduced compression ratio, and thus, a lower reading (relatively) on the compression gauge.

    E1. Just like the connecting rod, small tolerance variations in individual pistons means, generally, no two pistons will ever achieve the EXACT same compression reading, in the same engine, at the same time. (Again this relates to manufacturing tolerances. In precision manufactured modern engines from major manufacturers this is not an issue. In our "loose tolerance" machined China Girls, careful selection out of a batch of the "same" part will result in the ability to assemble engines with much finer tolerances from parts that are otherwise not very consistent from part to part. This is what seperates a skilled engine builder -among many things- from someone who just knows how to assemble all the parts together in the right order.)

    D. Other Variances. There are other variances that play a role, such as the free-play up and down in the big and small rod end bearings, and their pins, as well as free-play in the crank bearings in the case. The case casting itself, and how low or high the cylinder mounting face is to the crank axle center-line. The higher (relatively) this face is in relation the crank axle center-line, the higher the cylinder will then be. The higher (relatively) cylinder now results in a taller deck height, increased combustion chamber volume, lower compression ration, and thus, a lower (relatively) reading on the compression gauge.

    5. The Condition of the Rings. Brand new rings will not give as high a reading (relatively) on the compression gauge as rings that have been properly worn in. Once the rings are sufficiently worn in they will provide their maximum available compression reading. After this point the compression reading will steadily drop as the rings wear, and more and more blow bye is allowed past the rings, reducing compression. As a VERY GENERAL rule of thumb: Once the reading you get on the compression gauge is 20-25% smaller than the reading you got when you installed brand new rings, it is time to freshen up the top end. For us that means new cylinder and rings, I will usually re-ring the piston once before I replace the cylinder though.

    6. The Pre-MiX Ratio used for the Oil/Gas Mix. Simply put: The more oil you mix in your gas, for example: 20:1 vs 50:1, the higher the reading you will see on the compression gauge.

    Short Answer:

    As you can see, due to the different types of parts that are used to assemble our lovable China Girl engine, as well as the various "loose tolerance" variances involved in its manufacture it is literally IMPOSSIBLE to tell you what your engine "should" achieve in a compression test.

    The best I can do is give you general guidelines.

    If it reads below 80psi on a brand new engine, don't expect amazing performance, but it should totally start as long as it has somewhere around 40-50psi... and there isn't something drastically wrong inside the cylinder.

    Anything from 90-120psi is smiles all day long, good stuff for a stock engine, and not too shabby for a mildly upgraded engine either.

    120-150psi is performance engine territory, definitely have to keep an eye on engine temps, and it would generally be recommended to keep the oil mix at 32:1 or richer (i.e. 24:1, 20:1, 16:1).

    150psi+ is extreme performance engine territory, and you had better know what you are doing.

    I run my engines anywhere from 230-250psi, but they are strictly race engines, they are not run on the street, only at race events. I run a big Jake Head, 16:1 oil mix, 0.025" squish gap. I tune the ignition advance and the carburetors judiciously, run oxygenated race gas (oxygenates help cool the cylinder) and I keep a SHARP eye on my spark plugs.

    After all of this, I get roughly 10-20 hours of run time before I have to rebuild the top-end... If it makes it that long.

    Luckily I always have a new porting idea I want to try. When I finally tackle exactly what I want out of the port arrangement I am going to pony up for some Nikasil. Gotta love that stuff.

    .... I could go on for a few more pages on the subject, but I think I'll leave it here for now. If anyone has anymore question don't hesitate to ask... I like typing.

    Sean Davis
    Apollo Moto Racing
    MotorBicycleRacing likes this.
  2. Eagle420

    Eagle420 New Member

    Feb 10, 2019
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    Heya Exo,

    I recently purchased my first gas bike kit - a Grubee Skyhawk GT-5A 66cc bike kit, preinstalled on a rusty but solid OLD mountain bike. Got it for $150 Canadian which seemed fair.

    After spending hours learning, researching, cleaning up the old engine, and troubleshooting, I found out the stock exhaust was SOO CLOGGED UP, it was spewing gas mist out the air filter...

    Ran it BRIEFLY with no exhaust and it ran like a champ, albeit loud AF.

    Once I determined the exhaust was indeed faulty, I ordered a cheap banana pipe from China (up to 2 months shipping) and drilled some holes into this c0cksucker exhaust. Runs well enough now and has been my daily driver for approx. a week.

    Still learning lots. Drained the gross old gad (this bike was sitting all winter), put in some "silver" grade gas (89 octane), mixed properly 32:1 ratio for now. Adjusting carb needle for our colder temperatures in the winter, getting LOTS of 4-stroking going on, trying to find the sweet spot and thinking about trying different jet sizes. Also I taped the gaps on the prongs of the stock carb, as they were letting in SOME unfiltered air. Had to remove the carb clamp and taped underneath where it sits (on outside of carb).

    After all this, I finally bought a compression test kit, along with lithiun grease, WD-40, NGK spark plugs, etc., expecting to see low compression on this stock kit. I even ordered piston rings for $5 prematurely, and Zoom Bikes nearby in Canada here hooked me up with a couple surprise free parts like a spare piston, wrist pin bearings, and ither things the fella thought I might need for a used engine.

    Wayyy better experience with them (Zoom Bikes), than with Gasbikes.net. I ordered 3 things with expidited shipping from Gasbikes, and they took over a week to even SEND the items to USPS. Took over 13 phone calls from 4 phone #s to talk to a live agent more than once, and nobody knew what was going on, or why the package was not shipped. It seems they did send it that day (over a week after I placed an order). What a joke.

    After all that, my new CDI arrived but does not match the magneto components (see here: http://www.grubeeinc.com/Magneto Mystery.html). I have SD parts and they sent TZ CDI... They didn't bother to ask, and I was not educated enough to ask.

    Fast forward to a couple days ago. I did NOT warm the engine up fully (it was at room temperature) when I hooked up Compression Test Kit. Took it outside BRIEFLY, biked between 5-10km/hr, and was astonished to see 180psi!...

    From what I understand now, this seems concerning.

    I only use it for going to work and back, approx. 10-15 minutes of riding and 3km approx. to get from home to work.

    Should I be concerned with this high compression rating? If so, what would you recommend? I also ordered a set of gaskets from Zoom Bikes, and my old gasket seems good enough; i can MAYBE slap on a 2nd gasket to immediately reduce compression if needed.

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